Single vs two-hose image questions?

[Nemrod]

James,

You've nailed it here. The current single hose regulators will not come close to the effortless exhalation of the Scubapro Pilot or A.I.R. I regulators. For these two regulators, the entire exhaust diaphragm is the exhalation valve.

Akimbo mentioned work of breathing on the rebreathers. My understanding is that there is no Venturi concept,

Now, about double hose regulators, the Venturi on most later double hose regulators was such that they would almost breath for themselves, and needed backpressure to turn off (stopping of inhaling initiates the backpressure), as mentioned in an earlier post. This means that it is mostly the cracking effort that is affected by the positioning.

SeaRat

1. Poseidon Jetstream . However, any second stage that has the large 30 mm exhaust mushroom valves (example SP G250/late USD 1085 or the large oval shaped valves (example Atomic) will have a low exhaust effort. It is the little 25 mm valves and smaller that are inhibiting. The manifold the valve sits in, the shape of the tee and the case geometry also have an effect upon the exhaust effort in addtion to the valve diameter. So, you might think, 25mm vs 30 plus mm, no big deal, just a difference of 4mm or 5mm, no big deal right! No, it is huge, the area of a circle increases dramatically, do the math, as the circle become even slightly larger, it is day and night IMO, well, actually it is not an opinion, it is measurable on my Magnehelic. For the G250 SP specifies .4 to .6 inches for exhaust effort and I get that easily with my G250s but with the "pretty face" well, it is a little unfortunate. But it is pretty. That counts!

2. I did an Intro to CCR some many years ago in Phoenix. It was not a full course. We went through the physics, setup and maintenance of the units and fooled around with them in the class room. Then we got in the pool. It was a deep enough pool built for dive training. Breathing through the unit was like sucking molasses. I have some sort of anxiety around having any pressure on my chest or any feeling I am not breathing freely. It is a phobia and the CCR set it off big time. I finished the little intro course and decided that in addition to the technology being immature I was not impressed with the breathing and was not interested in an early arrival at the Pearly Gates.

3. Yes, very succinct, positioning mostly affects the cracking effort and once the Venturi is established a well tuned regulator like the Argonaut will push the air to the diver. The HPR valve in the Argonaut can be tuned by regulating the bleed ports to the point that the regulator bypasses air through the loop at each breath from the aggressive Venturi. There is a vane in the mouthpiece (of some Argonauts) to help direct the air to the diver and reduce bypassing. This allows a more aggressive tune than would otherwise be possible.

James

 

[Luis H]

I was thinking more about this after our phone conversation last night. Perhaps the most important reason that perceived WOB is more important in inexperienced recreationally divers is panic. It is well know that respiratory resistance and constriction is a panic-inducing stressor.

That applies to the breathing circuit as well as constriction by poor fitting wetsuits or suit-squeeze by under-inflated drysuits.

I agree that the perceived Work of Breathing is important to some inexperience divers, but the perceived WOB is not just important for beginner divers. It is very important for experienced Navy divers, for technical diver, and for all recreational divers. It is what the Navy measures with the test setup that I have witnessed and as far as I have seen it is also what it is measured in the dive labs all over.

For the benefit of others, let me clarify that perceived WOB is not something imaginary or that our senses are getting fooled or anything like that.

In human engineering it is well known that our human perception is extremely important, but we are not a measuring instrument. Therefore, we have to design equipment to fit the human, not adapt or train the human to fit the equipment.

Our interface to the world is what we perceive. For example, we think we can measure temperature, but we don’t. What we actually feel is, if something is hot or cold, relative to our warm skin temperature. You can touch a bar of soap and a bar of aluminum. The aluminum will feel colder even if they are at the same temperature. We humans, measure heat transfer not temperature.

As I mentioned, the test dummy used at NEDU look like the torso of a “crash test dummy” and it is instrumented on the back of the throat, as I mentioned on my previous post. I can guaranty you that the Navy is not spending a lot of money, time, and effort just for the comfort level of a beginner diver.





About the subject of exhaust mushroom valves, it is not just the size of the valves that matter. There have been a lot of work and design improvements on mushroom valves. A lot of it may seem like small changes and the casual observer could not tell much of a difference by their looks.

Some of you will remember the old flat exhaust valves. The geometry was just a flat disc and their stiffness was mostly driven by material thickness and material properties, softer rubber softer valves.

Take a closer look at some of the modern mushroom valves. They are not the same and the geometry from one to the next is very different. Some have spokes; some have a circumferential kink at different radiuses and with different angle of kink. The material thickness is often not uniform anymore.

One important thing all the new mushroom valves seem to have in common is that they only touch and seal on the outer perimeter of the valve. There are lifted away from any supporting (wagon wheel) structure to avoid the water surface tension.

I have seen the computer FEA (Finite Element Analysis) models on some of this modern valves and the engineering development is very impressive. The geometrical structure of the valves allows for intended elastic buckling to open up, not just elastic flexing.

To the casual observer it is not at obvious the amount of work and development that has taken place over the years on what we consider a “simple mushroom valve”.

The modern mushroom valve has very close to zero (not measurable) initial opening effort (unless it is sticking due to dirt). And the valve itself tends to move out of the way with flow. The flow resistance is mostly a function of the open area.

This valves are used in many applications (not just Scuba), but again the rebreather need has driven some of the technology (at least for Scuba).

One of the things that impressed me the most was the level of detail in the FEA model and engineering detail for a “simple mushroom valve”. The model was way more involved and sophisticated than some of the FEA we did 40 years ago for spaced deployed equipment (including some parts in the Hubble Optical telescope). Times have change…

 

[Akimbo]

It is very important for experienced Navy divers, for technical diver, and for all recreational divers.

I didn't mean to imply otherwise except that navy (any navy), commercial, tech, and experienced recreational divers are MUCH less likely to panic in stressful or uncomfortable situations underwater.

 

[Luis H]

My post was mostly for clarification, for the benefit of others. I knew what you meant.

 

[Akimbo]

My post was mostly for clarification, for the benefit of others. I knew what you meant.

It is a very important point that I should have made clear in the first place. It is easy to forget that we aren't having a conversation between buddies. Thousands of people from old salts to wannabe divers will read this over the years.

 

[Gone for diving]

Here is an image of Jacques Yves Cousteau gearing up to test dive a "push-pull" demand regulator for the Conshelf 3 experimental saturation dive in 1965.

View attachment 693768​

It allowed the expensive Helium-Oxygen breathing gas to return to the habitat for reprocessing. Note that it was a chest-mounted double-hose configuration.

The same concept is used today in sat diving except the demand exhaust regulator is mounted to the hat and the exhaust is routed through the bell and back to the surface for reprocessing.

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Can you explain how it is recirculated?
I Assume the bell needs to be higher then the diver, but that will cause a decrease in pressure in the helmet... is there some kind of reverse demand valve?

I assume the vintage and modern systems are similar?

 

[Akimbo]

Can you explain how it is recirculated?

It is basically a giant surface-based rebreather with huge multiple banks of HeO2, virtually unlimited power, and multiple professionals to monitor it.

The supply side of the system is unchanged from old open circuit systems. Gas is supplied from tube-trailer size cylinders onboard the DSV (Diving Support Vessel).

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Then it passes through:

• The bell control console where Oxygen is analyzed and distributed.
• Into the gas supply hose in the bell umbilical
• Inside the bell and the diver gas supply manifold
• Into each diver's umbilical supply hose
• Into the diver's hat with a demand regulator and freeflow valve.

The diver exhales and the gas passes through:

• A hat-mounted demand exhaust regulator
• Into the diver's exhaust hose in the umbilical
• Into the diver's exhaust manifold inside the bell, which usually has a negative biased back pressure regulator (like a first stage in reverse) and water separator(s).
• Out of the bell through the bell exhaust hose
• Into a gas recirc system on the ship with water separators, filters with CO2 absorbent, a volume tank with Oxygen sensors and automatic O2 addition.
• To the HP booster compressor's intake
• Through more water separators
• Past analyzers with alarms to verify the proper mix
• Into the gas distribution manifold
• And finally back into one of the HeO2 gas banks, not necessarily the on that is online at the moment.

You might wonder why commercial sat diving operations go to all this expense instead of just using back-mounted rebreathers. The simple answer is reliability and time.

DSVs work 24/7 and are often offshore 11 months between yard periods. If anything on the ship fails and divers can't work, the ship goes off contract — which is tracked by the minute. The customer eats weather down-time under most contracts, but it better be on par with competing DSVs or the contract doesn't get renewed.

A DSV like this leases for hundreds of thousands of US dollars per day all-in.

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Would you gamble more than more than $10,000/hour on recreational rebreathers used under these conditions?

 

[Luis H]

I would like to know more about these two items:

• A hat-mounted demand exhaust regulator
• Into the diver's exhaust hose in the umbilical

What is the pressure in exhaust hose umbilical?

Do you have any links or reference for more information on the "hat-mounted demand exhaust regulator"?

Thanks



BTW, anyone interested in this subject in general, the movie/ documentary Last Breath is great!

 

[Gone for diving]

Into the diver's exhaust manifold inside the bell, which usually has a negative biased back pressure regulator (like a first stage in reverse) and water separator(s).

This is what I was wondering about,,,
Reverse first stage,

Seems like they are not using a very big hose to return the exhaust, so there must be quite a pressure differential to help, which could be a problem really fast if something goes wrong on the valve,

Thanks for the detailed description,

 

[Akimbo]

I would like to know more about these two items:

• A hat-mounted demand exhaust regulator

There are two major manufacturers, Fisher/Divex Ultrajewel and Kirby Morgan. Here is documentation on the Kirby Morgan Diamond Exhaust Regulator Maintenance and Testing.

You can find this level of documentation on all the Kirby Morgan gear, See Manuals and BlowAparts

What is the pressure in exhaust hose umbilical?

It varies a bit but typically in the minus 1.5 to 2 ATA range at the bell and increases by excursion depth below the bell. Most operators don't like to have the bell farther from their divers than required but structures can interfere. There are the obvious safety reasons but also to save time and bell lights help illuminate the worksite.

This is what I was wondering about,,,
Reverse first stage,

We used to use the Tescom and AFAIK, it is still the most popular.

TESCOM BIBS NEGATIVE BIAS; BACK PRESSURE REGULATOR; DOME

Seems like they are not using a very big hose to return the exhaust, so there must be quite a pressure differential to help, which could be a problem really fast if something goes wrong on the valve,

We used 1/2" or 12mm exhaust hoses in the diver's umbilical and 3/4" or 20mm in the bell umbilical. The bell umbilical had to handle two divers when locked out and any venting of bell pressure when the hatch was closed — typically less than 150'/45M between the excursion depth and the sat holding depth. The bell umbilical usually saw about -3 ATA relative to the bell.